Asthma is one of the leading causes of morbidity in children with 60% of all cases diagnosed by age 3. Thus, finding factors that can lead to prevention of this disease would be of great public health importance. Vitamin D deficiency is highly prevalent among pregnant women, and thus, represents a potentially modifiable factor for the prevention of disease. Due to the effect of vitamin D in modulating immune responses, we believe that vitamin D deficiency in pregnant mothers leads to faulty immune system development in the neonate, predisposing them to asthma and allergy. We have observational data from two independent birth cohort studies that higher maternal intakes of vitamin D during pregnancy are each independently associated with a 50% reduction in risk for recurrent wheezing and asthma in 3- and 5-yr old children. However, in order to recommend this as a universal treatment to prevent asthma, a randomized, controlled, clinical trial is necessary. Therefore, we propose a two arm, double-blind, placebo controlled, randomized, clinical trial of Vitamin D, to determine whether higher vitamin D intake and levels in the pregnant mother will prevent asthma and allergy in the child at age 3. We will identify pregnant women who have asthma or allergies or whose partner has asthma or allergies, from obstetric clinics in the three clinical centers participating in this trial. We will recruit 870 pregnant women during the first trimester of pregnancy and randomize them to one of two treatment arms of a 4-year clinical trial: 4000 IU of Vitamin D in addition to usual prenatal vitamins, n=435; and usual prenatal vitamins alone, n=435. Our primary specific aim is to determine whether adequate vitamin D supplementation in the pregnant mother is associated with reduced incidence of asthma in the child during the first 3 years of life. The sub-aims of the study will include (1) To determine whether adequate vitamin D supplementation in the pregnant mother is associated with reduced secondary outcomes in the child of (a) allergic sensitization, (b) doctor's diagnosis of eczema and (c) lower respiratory tract infections during the first 3 years of life; and (2) To determine whether adequate vitamin D supplementation in the pregnant mother is associated with improved vitamin D status in the mothers and their offspring through measurement of 25(OH)D levels in maternal plasma, cord blood, and children's blood at 1 and 3 yrs of age. Public Health Relevance: We believe that it is likely that Vitamin D deficiency has had a significant impact on the asthma epidemic that began in the early 1980's. Greater time spent indoors, and use of sun screen has resulted in reduced sun exposure and decreased production of vitamin D; and lack of adequate dietary supplementation to compensate for these behavioral changes, results in very low vitamin D levels. We believe that with adequate dosing of vitamin D in pregnant women, we can prevent half of all asthma cases in their offspring and ultimately reduce disease prevalence to pre-1980 levels. If successful, this trial will ultimately decrease asthma incidence by 50% and consequently decrease health care costs by 8 billion dollars. This intervention is cheap, safe, and will result in major public health benefit. It also represents the most innovative and novel approach to asthma treatment and prevention in the last 30 years.